Manganese zinc ferrite core



Patented May 8, 1951 UNITED STATES PATENT OFFICE MANGANESE ZINC FERRITE. CORE Conn., as trustee No Drawing. Application April 8, 1946, Serial No. 660,420. In the Netherlands July 1, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires July 1, 1963 8 Claims. 1

This invention relates to a magnetic core containing a cubic ferrite as magnetic material and to the manufacture of such a material.

According to the invention a mixed crystal of manganese ferrite and zinc ferrite, hereinafter called manganese zinc ferrite, is used to constitute the magnetic ferrite material.

A manganese zinc ferrite according to the invention has the advantage that it can be manufactured with a high value for the initial permeability.

Furthermore the use of manganese zinc ferrite according to the invention permits the manufacture of magnetic material whose temperature coeflicient of the reciprocal initial permeability, in other words the temperature coefficient of the magnetic reluctance, in a temperature range in the proximity of room temperature, say of from 10 to 40 C., has a substantially constant negative value. Over such a wide temperature range for the temperature coefficient of ferrites other than manganese zinc ferrite solely a positive value could hitherto be obtained, but now it is possible, by combining such a ferrite having a positive temperature coefficient with a manganese zinc ferrite according to the invention having a negative temperature coefficient, to design a magnetic circuit having a temperature coefficient of zero or practically zero. Such a magnetic circuit has the advantage that the inductance of a coil cooperating with this circuit does practically not change in the event of temperature variations.

Sometimes it has proved possible to prepare a manganese zinc ferrite having such a low temperature coefficient that the material in itself ensures an inductance value which is independent in regard to temperature.

To make this more clearly understood it is pointed out that the temperature coefiicient of the reciprocal initial permeability and not that of the initial permeability is of technical importance, since in a magnetic circuit comprising one or more air-gaps the magnetic reluctance is additive.

Another advantage resulting from the use of manganese zinc ferrites isthe possibility of obtaining a magnetic material which, in the presence of low inductions in an extensive range of frequencies, up to 100 or even 1000 kilocycles/sec. and higher, has low total losses (eddy current losses, hysteresis losses and other losses), which is of importance for use for radio, telegraphy, and telephony purposes, electro-acoustic devices and so on.

Furthermore the use of manganese zinc ferrites permits a material to be obtained having particularly low hysteresis losses, which is of importance in telegraphy and telephony for transformers, Pupin coils and so on.

In addition, an important point consists in the possibility of preparing a manganese zinc ferrite having several or all of the aforesaid favourable properties.

As regards the preparation of a manganese zinc ferrite according to the invention the following may be said.

Preferably an intimate mixture of the pure metal oxides building up the ferrite is obtained either by mixing the separate oxides or by precipitating, with a base, a solution containing manganese, zinc and iron; in the lastmentioned case the precipitated oxyhydrate mixture may partly have the ferrite structure already. An alternative method of preparing the ferrite is to heat the carbonates of the metals which upon heating convert into the oxides thereof. The starting mixture is preferably compressed and subsequently sintered until it passes over into ferrite.

For obtaining a high initial permeability it is desirable to take care in sintering, for instance by a. sufficient temperature or a sufficient heating time, or by a sufficient reactivity of the ferrite forming starting mixture, or by grinding the first sintering product and by sintering anew, or by a combination of these steps, that the formation of ferrite from the starting constituents should take place as thoroughly as possible i. e. that the mixture thoroughly reacts. Furthermore, to promote a thorough reaction use is preferably made of a mixture having a great fineness in preparing a manganese zinc ferrite. To obtain a great fineness and, moreover, a high reactivity the starting mixture may be ground for a long time and very intensively, and preferably to such a degree as to attain an average size of the particles of less than 1 Mixtures of oxides or oxyhydrates, obtained by wet process, by precipitating a solution of the metals in question with a base have a great fineness.

The ratio of the constitutents of the starting mixture, which may either be stoichiometric or depart therefrom, is chosen in such a manner that at the sintering temperature the mixture can pass over into a practically single homogeneous ferrite mixed crystal phase, the contents of iron oxide usually amounting of from 40 to mol. per cent. The mixing ratio of manganese and zinc is preferably so chosen as to form a mixed crystal having a Curie-point between 3 about 40 and 250 0. Such a Curie-point is advantageous in View of attaining a high initial permeability. It is pointed out that. also the iron content and the heat treatment of the ferrite acts upon the Curie-point. In the present case the expression Curie-point is to be understood to mean the temperature at which a magnetic material passes over into a state which in regard to the permeability may be conceived to be nonmagnetic for practical purposes. It is still to be noted that without departing from the principle underlying the invention it is also possible to combine oxides, other than manganese oxide and zinc oxide, with iron oxide and the expression manganese zinc-ferrite also includes the ferrites thus obtained.

For the obtainment of a material having. a substantially constant negative or very low temperature coefficient of the reciprocal initial permeability over a comparatively wide temperature range it has proved desirable to carry out the sintering operation of the ferrite forming starting mixture in oxygen or air or an atmosphere giving on a sufficient quantity of oxygen and to cool the formed ferrite, after sintering, in such an atmosphere at a definite rate. Good" results are obtained by cooling at a rate of about 5 C. per minute, The optimum cooling speed depends on the conditions such as the temperature at which the sintering operation has taken place, on the composition of the ferrite, on possible impurities, on the temperature coefficient to be attained, but it can be easily found experimentally.

Ithas turned out that when preparing a manganese zinc ferrite in the aforesaid manner the obtained ferritehas in addition to a favourable temperature coefficient a very low' value in regard to the losses. Lo'W lossvalues can also be obtained when cooling more rapidly, but such a rapid cooling initiates detrimental quenching strains which render the material brittle and adversely affect the initial permeability. In view of the permeability a cooling speed of about 5 C. per minute is advisable.

A judicious choice of the conditions, according to the invention, permits the obtai'nment of magnetic material having a loss factor we which in the case of low inductions over a wide frequency range, up to 100 or even 1000 kilocycles/ sec.' has a very low value, for instance smaller than 0.06. The characteristics tgo corresponds to where It represents the loss: resistance, measured whilst avoiding. the occurrence of dielectric losses and after deduction of the direct current resistance, and L represents the inductance of a coil which is wound on an annular core consisting of the ferrite material, a: being the circle frequency. A material having such low losses is very suitable for radio, telegraphy and telephony purposes.

When taking at the same. time, in preparing a manganese zinc ferrite involving low losses, the measureslpromoting. the attainment of a high initial permeability a particularly valuable magnetic material can be obtained, since the usefulness of a magnetic core for high frequencies is mainly determined by the small value of the quotient of the aforesaid loss. factor tga and the initial. permeability [L measured with the annular cor According to the invention a material can be made with which t g p.

is. lower than 10- at frequencies up to kilocycles/sec. Such a material is eminently suitable for the manufacture of filter coils for telephony purposes, which will mostly be used at frequencies of from 10 to 100 kilocycles/sec. However, the material according to the inventionis: also verysuitable for Pupin coils that are used at frequencies of from 300 to 2000 cycles/sec. To make thi better understood it is pointed outthat this material constant is a suitable characteristic for judging a magnetic material, since in a magnetic circuit having one or more so-called air-gaps the quotient of of theefiective loss factor tgoerr and the effective initial permeability [.Leff, with a constant magnetic load of the material, does not depend on the. number and the size of the air-gaps and consequently corresponds to where too and. a have been measured with an annular core.

The ,ueff is to be understood to mean the value following from the well-known expression:

1 1,- 1,- 6 Me i 1- for a magnetic circuit having one or more air gaps.

The value of the quotient once being determined for a ferrite, for instance by measurements withv an annular core of the ferrite, the loss factor of another circuit built up from this. material can be found by multiplication of the constant by the effective permeability of this circuit. 7

As has been stated a considerable advantage of the use of manganese zinc ferrites consists in the possibility of obtaining a magnetic material having low hysteresis losses. According to. the invention such properties are obtainablev more particularly if the content of. iron oxide in the manganese zinc ferrite amounts to 52 mol. per cent or more. Very good results. are obtained if the content amountsv to 52 to 55 mol. per cent, but. it. i also possible to use a higher content, inasmuch as, ferrite mixed crystal having such a high iron oxidecontent is possible. Ferrites having. low hysteresis. losses are. of much importance for telephony purposes.

At the present moment it is not yet possible completely to account for the remarkable magnetic properties which render manganese zinc ferrite particularly suitable for magnetic cores, but. it is probable that they are related with the fact that. manganese may occur in diiferent valency stages and on the occurrence of temperature variations may pass over into a different. valency stage whilst absorbing, or giving off oxygen.

It. is still pointed out. that the expressionmagnetic core in the present specification includes not only a core within a coil, but generally parts of electro-magnetic constructions used in view of their magnetic properties, for instance also magnetic shielding parts.

Examples 1. A mixture of technical zinc oxide, manganese dioxide and iron oxide in a mol. ratio of 23.5:23.5:53 reckoned in regard to the pure oxides, ZnO, Mn3O4 and F8203, is ground for 12 hours in an iron ball mill. The mixture contained about 0.7% silicium dioxide as main impurity. Good results were also obtained with a content of about 2.5%. The mixture is moulded to form a ring having an internal diameter of 2.5 cms. and a cross section of 5 to 5 mms. at a pressure of 4000 kg./cm. with water as a plastification agent and binder. This ring is sintered for two hours at 1300 C. in an electric furnace in oxygen. After that the furnace is switched off and together with its contents is allowed to stand, the passage of oxygen over it being continued. In about three hours the ring cools down to 200 C. The obtained manganese Zinc ferrite had a Curie point of 116 C. and an initial permeability 415, measured at 20.

The temperature dependency of the initial permeability is small so that the inductance of a coil made from the material, with an effective permeability of 15, from 20 to 50 C. is constant with an accuracy of less than 0.15%. At 20 C.

the temperature coefiicient was positive. The values of are stated for various frequencies in column 2 of the followng table. At a frequency of 2000 cycles/sec. and a maximum induction of 7.5 Gauss the hysteresis factor amounted to 11, Rh representing the hysteresis resistance of a coil wound on the annular core and Z representing its inductance thereof.

2. Similarly to Example 1 a core is made from technical oxides with a manganese zinc ferrite containing 51 -mol. per cent of iron oxide and equal percentages of manganese and zinc. The initial permeability amounted to 335 and the Curie point to 82 C. The temperature dependency was low; at 20 C. it was negative. The values of are stated in column 3 of the table.

amounted to 90 at 2000 cycles/sec. and a peak induction of 7.5 Gauss.

3. A mixture of pure manganese dioxide, obtained by roasting manganese nitrate, pure zinc oxide and pure iron oxide in a mol. ratio of 25:21:54. is ground for 12 hours in an iron ball mill and after than moulded to form a ring and sintered in the manner set out in the first example. The obtained initial permeability amounted to 470 and the Curie point to 124 C. The temperature coefficient had the same value as in the first example but at 20 C. it is negative. The values for are stated in column 4 of the table. At 2000 cycles/sec. and a peak induction of 7.5 Gauss amounted to 3.0.

4. A mixture of the oxides stated in Example 3 in a mol. ratio of 32:20:48 is worked up into an annular core in the aforesaid manner. Its initial permeability amounted to 590 and its Curie point to 118 C. The temperature coeflicient was very low and at 20 C. positive. The values for are stated in the fifth column of the table. At 2000 cycles/sec. and a peak induction of 7.5 Gauss For the purpose of defining the terms ferrite, "mixed crystal, and soft magnetic materials, the following definitions will be employed in connection with the above-noted terms as used throughout the specification and in the appended claims:

A ferrite is a crystalline material which is a compound of the reaction product of a metal oxide and iron oxide having the empirical formula MF204 wherein M represents a bivalent metal. This material may also be defined as a metallic salt of the hypothetical acid H2F2O4.

A mixed crystal ferrite is a ferrite material comprising two or more ferrites as hereinbefore defined which are chemically combined together to form a single homogeneous crystalline compound.

The term soft magnetic material means magnetic material having a low remanence and a low coercivity when the applied inductive field is removed from the material.

Within the scope of the definitions noted above, we have described our invention with specific examples and methods of execution, which, however, will suggest other obvious modifications to those skilled in the art without departing from the spirit and scope of our invention.

What we claim is: Y

1. A method of manufacturing a soft ferromagnetic material comprising the steps of forming a mixture consisting of about 40 to 57 mol. per cent of F6203, about 23 to 32 mol. per cent of MnOz, and the balance ZnO in an amount of at least 20 mol. per cent, the said oxide being in the proportions producing at the sintering temperature a practically single. homogeneous, ferrite phase, sintering the mixture in an atmosphere containing a substantial amount of oxygen and at a temperature greater than 1000 C. to form mixed crystals of manganese-zinc ferrite and cooling the manganese-zinc ferrite in the order of about 5 C. per minute in the said atmosphere.

2. A method of manufacturing a soft ferromagnetic core material comprising the steps of forming a mixture of powdered manganese dioxide, powdered zinc oxide and powdered ferric oxide, the amounts of said oxides being in the relative proportions producing at the sintering temperature a practically single homogeneous ferrite mixed crystal phase and lying within the limits of about 24.5 to 32 mol. per cent manganese dioxide, about 20 to 24.5 mol. per cent zinc oxide and about 48 to 54 mol. per cent iron oxide, sintering the mixture in an atmosphere containing a substantial amount of oxygen and having a temperature greater than about 1000 C. to form mixed crystals of manganese zinc ferrite and cooling the manganese-zinc ferrite in an atmosphere containing a substantial portion of oxygen at a rate of about 5 C. per minute.

3. A method of manufacturing a soft ferro magnetic material comprising the steps of forming a mixture of powdered manganese dioxide, powdered zinc oxide, and powdered ferric oxide, the amounts of manganese dioxide and zinc oxide being in equi-molecular proportions and the ferric oxide comprising between about 51 and 53 mol. per cent of the mixture, sintering the mixture in an atmosphere containing a substantial amount of oxygen and having a temperature. greater than about 1000 C. to form homogeneous mixed crystals of manganesezin.c ferrite and cooling the mixed crystals of manganese-zinc ferrite in an atmosphere containing a substantial portion of oxygen at a rate of about 5 C. per minute.

4. A method of manufacturing a soft ferromagnetic material comprising the steps of forming a mixture of about 21.0 mol. per cent of powdered zinc oxide, about 25.0 mol. per cent of powdered manganese dioxide and about 54 mol. per cent of powdered iron oxide, sintering the mixture in an atmosphere containing a substantial amount of oxygen and having a temperature greater than about 1000 C. to form homogeneous mixed crystals of manganese zinc ferrite and cooling the mixed crystals of manganese zinc ferrite thus obtained in an atmosphere containing a substantial portion of oxygen at a rate of about 5 C. per minute.

5. A method of manufacturing a soft ferromag- 8 netic material comprising the steps of forming :a mixture of about 20.0 mol. per cent of powdered zinc oxide, about 32 mol. per cent of powdered manganese dioxide and about 48 mol. percent of powdered iron oxide, sintering the mixture in an atmosphere containing a substantial amount of oxygen and having a temperature greater than about 1000 C. to form homogeneous mixed crystals of manganese zinc ferrite and cooling the mixed crystals of manganese zinc ferrite in an atmosphere containing a substantial portion of oxygen at a rate of about 5 C. per minute.

6.. A soft ferromagnetic material consisting of substantially homogeneous mixed crystals of manganesazine ferrite having a Curie point between about 40 and 250 0. prepared in accordance with the method claimed in claim 1.

7. A soft ferromagnetic material consisting of substantially homogeneous crystals of manga-. nese-zinc ferrite having a Curie point between about 40 and 250 C. and a loss factor tan cless than 0.06 below kc./sec. prepared in accordance with the method claimed in claim .2.

'8. A soft ferromagnetic core material consisting of substantially homogeneous mixed crystals of manganese-zinc ferrite having a Curie point between about 40 and 250 C. and a quotient tan 5 ,a being the initial. permeability and tan 6 being the loss factor, of less than 0.0001 below 100 kc./sec. prepared in accordance with the method as claimed in claim 3.

JACOB LOUIS SNOEK.

JAN BOEKE.

MARINUS WILLEM LOUWERSE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 383,942 Conin June 5, 1888 1,651,955 Lowry Dec. 6, 1927 1,946,964 Cobb Feb. 13, 1934 2,388,664 Bandur et a1 Nov. 13, 1945 2,452,529 Snoek Oct. 26, 19.48 2,452,530 Snoek Oct. 26, 1948 2,452,531 Snoek Oct. 26, 1948 OTHER REFERENCES Comprehensive Treatise on Inorganic and Theoretical Chemistry, volume XII, pages 775- 777 and 785, J. W. Mellor; published by Longmans, Green and Co., 1932. 

1. A METHOD OF MANUFACTURING A SOFT FERROMAGNETIC MATERIAL COMPRISING THE STEPS OF FORMING A MIXTURE CONSISTING OF ABOUT 40 TO 57 MOL. PER CENT OF FE2O3, ABOUT 23 TO 32 MOL. PER CENT OF MNO2, AND THE BALANCE ZNO IN AN AMOUNT OF AT LEAST 20 MOL, PER CENT, THE SAID OXIDE BEING IN THE PROPORTIONS PRODUCING AT THE SINTERING TEMPERATURE A PRACTICALLY SINGLE HOMOGENEOUS FERRITE PHASE, SINTERING THE MIXTURE IN AN ATMOSPHERE CONTAINING A SUBSTANTIAL AMOUNT OF OXYGEN AND AT A TEMPERATURE GREATER THAN 1000* C. TO FORM MIXED CRYSTALS OF MANGANESE-ZINC FERRITE AND COOLING THE MANGANESE-ZINC FERRITE IN THE ORDER OF ABOUT 5* C. PER MINUTE IN THE SAID ATMOSPHERE. 